# Batteries: capacity, charging and discharing rate meaning

1. Mar 11, 2013

### fisico30

Hello Forum,

Battery chargers are devices to put energy into a secondary (rechargable) bttery by forcing an electric current through it. The charge current depends on the technology and capacity of the battery being charged.

The capacity of a battery represent the amount of electric charge (Coulomb) stored in the battery. The Coulomb unit is equivalent to Ampere-second.

A bucket of water has a certain volume and can contain a certain amount of water. The water capacity of the bucket is the same regardless of how fast or slowly we fill or empty the bucket. In the case of chemical batteries it does not work that way: the capacity of the battery depends on the charge/discharge rate. For instance, the faster we discharge a battery (large discharge current) the less the capacity that the battery offers.

I "think" that the capacity that is usually given for a battery tacitly implies a discharge rate of 1C. Is that correct? For instance, a 12 V battery has 1000mAh capacity. It means that at 1A drawn current the battery will last 1 hour and then will be completely discharged (or reach a certain terminal voltage < 12V). The implied discharging current is 1A.

Is this correct?

The charging can be different: to charge a battery from 0V to 12V we can use current of different magnitudes. The larger the current the faster the charging.
The rating for charging could be different for the same battery: 12V at 500 mAh. The 500mhA means that the battery will reach a total charge Q of 500 mAh if a charging current of 0.5A is used.....

In summary, the capacity (amount of charge storable inside the battery) depends on if we are talking about charging or discharing and on the magnitude of the current used....

So what is the max amount of charge we can store in a battery? Of course, that has to do with the charging phase. Does a small charging current lead to the max possible storable charge?

When looking at a battery specs, is it possible for the nominal voltage for the discharging be different for the discharging?

Thanks,
Fisico30

2. Mar 11, 2013

### Staff: Mentor

A couple clarifications...

Batteries do not store charge. They store energy in the form of chemical energy. That energy is released as you draw current out of the battery, and the energy is stored when you charge the battery.

The discharge time depends on the rate of discharge. Look at datasheets for representative batteries to see what I mean. I'll find one with Google Images and post it below.